The present invention relates to the co-extrusion of two or more streams of plastic material and the like, for introduction into a molding apparatus or similar devices. More particularly, the invention is directed to structure that enables better control of such co-extrusion, thereby providing for greater flexibility in the use of a wide range of suitable materials, extruding temperatures, and other conditions affecting the extrusion process.
With specific reference to injection systems for co-injecting at least two materials, the present invention relates to an improved technique, apparatus and resulting article for combining different annular flow streams of material where an interior layer of the combined annular flow stream can be terminated in a more abrupt fashion.
One conventional method of creating a multilayer object by co-injection molding is to inject annular layers of flowing material through a nozzle assembly into a mold. The result is a multilayer object having annular layers of material. The resulting multilayer object has an inner layer, an outer layer, and at least one interior layer sandwiched between the inner and outer layers. Depending on the end use requirements for the molded multilayer object it is often desirable to create a structure containing three or more annular layers of material. For example, in the case of a Polyethylene Teraphalate (PET) preform for a blown bottle, it is desirable to create a structure that contains three or more annular layers of material. The inner and outer layers of the preform are PET and at least one interior layer is formed from a material chosen to enhance the overall performance of the resulting plastic object, or to reduce the cost of the resulting plastic object. For example, interior layers may include one or more layers of a barrier material (MXD6 Nylon or EVOH), oxygen scavenging material, recycled material, or other performance-enhancing or cost-reducing material.
One problem in the field of co-injection molding resides in the need to end an interior layer of the material flow in a quicker or more abrupt manner. When molding a multilayer object it is often desirable to encapsulate the trailing edge of an interior layer of material with the inner and outer layers of material. The type of material used for the interior layer is often different from the type of material used for the inner and outer layers and as such, requires a region in the combining element extending from a stream combination area to a gate of a mold cavity to be free or clean of the interior layer material before the start of the next controlled volume shot of material into the mold cavity. This region must be free of the interior layer material so the inner and outer layer materials combine into a single encapsulating structure, or skin, that encapsulates the interior layer material. If this region is not free of the interior layer material the next controlled volume shot of material into the mold becomes contaminated with the interior layer material that remains in this region. Conventional nozzles for co-injection molding sequentially add layers of material to form a multiple layer output stream. As such, intermediate surface layers of conventional nozzles require cleaning, which is burdensome due to the sequential build of material layers.
Moreover, it is often desirable to have the interior layer material remain in close proximity to a base or gate portion of the resultant molded object. In the case of a PET preform, where the interior layer material can be a barrier layer, it is important to have the barrier layer extend as close as possible to the gate portion of the preform. Extending the barrier layer as close as possible to the gate portion of the preform results in a significant benefit when the preform is blown into a bottle. That is, a substantial portion if not the entire sidewall of the resulting bottle has the interior barrier layer. Absent a barrier layer that extends the entire sidewall length, the barrier property of the bottle can be adversely affected. The sidewall extends from a neck portion to a base portion of the resulting bottle. However, it is not always necessary for the gate portion of the bottle to include an interior layer as compared to the sidewall of the bottle, for the gate portion of the blown bottle tends to have a thickness sufficient to provide an adequate barrier to protect the contents of the bottle. Thus controlling a distribution of the material forming the interior layer of a molded object is important to the value and performance of the resulting molded object.
One conventional method of accomplishing this goal of controlling a distribution of the material forming the interior layer of a molded plastic object is to stop injecting the interior layer material into the mold while continuing to inject into the mold the inner and outer layer material. That is, when the flow of the interior layer material is stopped, the inner and outer layer material surrounding the interior layer material continues to flow dragging with them, in a downstream direction, the material that exited the interior layer material outlet of a combining element (e.g., nozzle assembly). In this manner, a stretching occurs between the interior layer material that remains substantially stationary in the outlet of the combining means and the interior layer material that has already exited the interior outlet of the combining element. This thinning eventually leads to breaking of the interior layer material from the combining element. Consequently, the inner and outer layer material makes contact once the interior layer material breaks from the combining means.
The breakage results in the formation of two interior layer components. The first is a trailing edge of the interior layer material just injected into a cavity of a mold. The second is a leading edge of the interior layer material which remains in the combining means for the next shot of material into the cavity. A further consequence of the breaking of the trailing edge of the interior layer material in this manner is the cleaning of such material from the gate of the combining element. As a result, the combining element is ready for the next injection cycle.
In the molded object, the structure of the trailing edge can be observed by coloring the discrete layers or by delaminating one or more layers of material after molding. Often, the trailing edge of the interior layer material has at least two observable regions. The first starts at the nominal core thickness of the molded object and quickly thins to an immeasurable thickness. The second is more burdensome to detect and can typically be detected by the lack of bonding between the inner and outer layers of material. This second region has only a microscopic quantity of interior layer material but it is enough to prevent the bonding of the inner and outer layer materials. In general, the first region accounts for ⅓ of the total tail length and the second region accounts for the remaining ⅔ of the total tail length.
FIG. 1 illustrates a partial cross section of a prior art three-layer co-injection nozzle assembly 200. Nozzle assembly 200 includes nozzle body 210, first nozzle member 212, second nozzle member 214, nozzle tip 216, and valve pin 218. Nozzle assembly 200 includes a third nozzle member (not shown) adapted to receive two or more material flows from respective material sources. Valve pin 218 in conjunction with second nozzle member 214 form inner flow channel 238 for carrying inner material stream 230 from an entrance orifice (not shown) to inner material egress orifice 226. First nozzle member 212 in combination with second nozzle member 214 form interior material flow channel 236. The interior material flow channel 236 directs interior material stream 232 from an entrance orifice (not shown) to interior material egress orifice 224. Nozzle body 210, first nozzle member 212, and nozzle tip 216 combine to form exterior material flow channel 240. The exterior material flow channel 240 directs an outer material stream 228 from an entrance orifice (not shown) to outer material egress orifice 222.
Nozzle tip 216, first nozzle member 212, second nozzle member 214, and valve pin 218 together define a combination volume 220. Combination volume 220 provides an area of combination where the inner material exiting orifice 226, the interior material exiting orifice 224 and the outer material stream 228 exiting orifice 222 combine to form combined output stream 234.
Outer material egress orifice 222, interior material egress orifice 224, and inner material egress orifice 226 are positioned in a common plane to provide combination volume 220 with three annular material streams. The three annular material streams flow substantially parallel to each other as each stream passes through each respective egress orifice 222, 224, and, 226 into combination volume 220.
Use of the conventional three layer co-injection nozzle 200, with an inner layer material stream 230 and an outer layer material stream 228 (i.e., skin) consisting of PET having an intrinsic viscosity (IV) of about 0.84 and an interior (core) layer material stream 232 consisting of MXD6 Nylon with a relative viscosity (RV) of about 2.65, the trailing edge or tail of the interior layer material in a molded preform with a 4 mm wall thickness often has a length of between about 15 mm and about 20 mm. One skilled in the art will recognize that the tail length of the interior layer material is a function of the viscosities of the materials used (i.e., skin and core materials) as well as the wall thickness of the molded preform. That is, as the preform wall becomes thinner, the tail of the interior layer material becomes longer in an inversely proportional relationship. For example, a preform with a 2 mm wall thickness formed with the conventional three layer co-injection nozzle 200 and the same core and skin materials identified above would have a tail length in a range of between about 30 mm and about 40 mm.
FIG. 2 illustrates an exemplary prior art preform 250 produced with the conventional three-layer co-injection nozzle 200. Preform 250 has an inner layer 256 and an outer layer 258 formed of PET having an IV of about 0.84 and an interior layer 252 formed of MXD6 nylon with a RV of about 2.65. The wall thickness of preform 250 is about 4 mm. As such, interior layer 252 has a tail 254 with a length of between about 15 mm and about 20 mm.